1. Field of the Invention
The present invention relates to communication systems. More particularly, the present invention relates to a system and method for controlling the true output power of a transmitter.
2. Background Information
Cellular telephones, Personal Communication Service (PCS) radio devices, and other communication devices use one or more power amplifiers to transmit power through an antenna to a remote receiving station, another communication device(s), or the like. However, in many such applications, the radio frequency (RF) power output of the communication device must be precisely controlled. For example, battery capacity may need to be conserved in portable devices when low power transmission is permissible or transmitted power may need to be maintained within acceptable governmentally-regulated limits at all times.
To control the power amplifier, a closed loop power control circuit can be used. In such a closed loop system, a power detector circuit can be used to determine a measure of the output power level of the power amplifier based on the output voltage of the power amplifier. Typically, the power detector circuit is designed using a diode-based detector, sensing the voltage at the output of the power amplifier. The diode-based detector detects the peak of the output voltage of the power amplifier. The detected peak voltage is compared against a predefined voltage threshold to generate an error signal. The error signal can then be used to control the output voltage level of the power amplifier. If the impedance seen by the power amplifier (e.g., an antenna or other load) varies little, then by controlling the output voltage signal level, such a method can control the output power of the power amplifier. By varying the predefined threshold, the output power level of the power amplifier can then be controlled by controlling the output voltage of the power amplifier.
The voltage signal at the output of the power amplifier is a function of the impedance of the antenna or other load. Consequently, controlling the output power of the power amplifier by controlling the output voltage assumes that there is little variation in the impedance seen by the power amplifier. However, with different impedances, the output voltage signal amplitude will differ for a given output power. Thus, precision of such a method in controlling the output power of the power amplifier based on an output voltage measurement is limited by the variation in impedance seen by the power amplifier resulting from, for example, component variation of off-chip matching network elements, impedance variations due to manufacturing, a change in the antenna ambient or the like.